A five point radial response surface design supplemented with a control group and several single factor points was utilized to investigate the effects of sodium bicarbonate and disodium phosphate on animal performance, ruminal fermentation, ration digestibility and rate of passage of liquid and particulate phases of digesta. Rations contained 0 to 4.5% sodium bicarbonate and 0 to 2.5% disodium phosphate.. Calves were randomly assigned to experimental rations, eight to the control group, six to the centerpoint, two to the radial points and three to the single factor points with restrictions as to time. Animals were on a growth trial from wk 7 to 14. Human samples were taken for determination of pH, buffering capacity, volatile fatty acids anct ruminal ammonia and blood samples for determination of glucose and plasma urea nitrogen at 8, 11 and 14 wk. Rate of passage was determined using ytterbium and cobalt EDTA to follow the particulate and liquid phases of digesta respectively. A digestion trial was carried out concurrently with rate of passage determinations. A two compartment model and a gamma time-dependent model were fitted to marker excretion data.

Average daily gains were not significantly affected by buffer additions, but appeared to be maxillial at 1 to 2% sodium bicarbonate and 0% disodium phosphate. Average daily gain and feed efficiency were not changed. Buffers consistently elevated total concentration of volatile fatty acids and osmolality in the rumen. Proportion of volatile fatty acids and acetate to propionate ratio were not changed by buffer addition. Digestibility of dry matter, organic matter, neutral detergent fiber, starch and acid detergent fiber are unchanged. Sodium bicarbonate increased ash digestibility while disodium phosphate tended to decrease ash digestibility. Urine volume and pH were increased by both buffers.

Rumen dilution rate for liquid digesta was increased by 50% and particulate digesta increased by 25% with addition of buffers. Flow of liquid digesta was best described by a two compartment model and particulate digesta by a gamma time-dependent model. No benefit of manipulating rate of passage in young dairy calves was observed in this study. Manipulation of ruminal fermentation by rate ot passage has potential to increase; milk production by bypassing starch and protein. Rates of flow of particulate digest may exert important affects on the ruminal acetate to propionate ratio and has potential to explain milk fat depression.